The present invention is generally related to radio frequency (RF) combiners coupling a plurality of RF transmitters to a single antenna and more particularly to a variable electrical length tuning stub and coupling loop interface for a ceramic transmitter combiner.
In order to combine a number of RF transmitters, the RF signals from each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. RF filters of the air-filled cavity type may be utilized to provide isolation between the RF transmitters. Each such cavity filter is tuned to pass only the RF signal from the transmitter to which it is connected, each RF transmitter producing a different frequency RF signal. The outputs from each filter may be combined and coupled to a common antenna by combining apparatus of the type shown and described in U.S. Pat. No. 4,375,622. Each filter is coupled to this combiner by precisely equal lengths of coaxial cable. The combiner is tuned by means of manually adjustable transmission lines or stubs, which likewise are coupled to the combiner by a coaxial cable. However, such combiners are not only difficult to tune, but also expensive and require an inordinate amount of precious space.